1. Field of the Invention
The invention relates to semiconductor microwave diodes and, more particularly, to more precisely defining the operating frequency of high efficiency GaAs Schottky barrier diodes having non-uniformly doped depletion region.
2. Description of the Prior Art
There is considerable interest currently in solid state microwave energy sources. Such sources promise to be more compact and less expensive, and to have considerably longer life than microwave tubes.
Among the most promising forms of solid state microwave sources is the impact-avalanche transit-time (IMPATT) diode disclosed in U.S. Pat. Nos. 2,899,646 and 2,899,652, which issued to W. T. Read, Jr. on Aug. 11, 1959. A characteristic of such a diode is a multi-zone semiconductive element which, when operating, includes a depletion region comprising an avalanche region and a drift region. A rectifying barrier, such as a Schottky barrier, contacts the avalanche region. A dynamic negative resistance is achieved by introducing an appropriate transit time to avalanching carriers in their travel across the drift region.
Early investigations have centered on increasing both the output power and the frequency of operation of these devices. More recent studies have concentrated on increasing the output efficiency, which, for GaAs IMPATT diodes, has typically ranged from about 10 percent to 15 percent of the input power.
It is now known that high efficiency (about 25 percent to 35 percent) GaAs IMPATT diodes may be obtained by more precisely defining the avalanche region. This is realized by forming a region of high doping level, or "clump" of charge, at a particular depth below the Schottky barrier contact, within that part of the body of the device that normally forms the depletion region when in operation. In this description, that region is termed the active layer. The location of the charge clump is dictated by desired operating frequency and efficiency considerations.
A continuing problem, however, has involved attempts to define more precisely the output frequency of the device. This frequency depends on the width of the depletion region, and is easily affected by material parameter variations. For example, calculations have shown that relatively small (about 5 percent) changes in impurity content and position of the charge clump result in unacceptably large (about 11 percent) deviations in the operating frequency.